1. Field of the Invention
The invention relates to the field of electromagnetic shielding structures for electronic units such as circuits, housings and subassemblies, using a flexible conductive envelope that encloses the electronic unit to prevent high frequency energy from passing into or out of a shielding boundary. More particularly, the invention concerns a flexible bag arrangement with access and conductor passages, suitable for the shielding of printed circuit cards of portable computers, notebook or palmtop devices and similar compact electronic devices requiring passage of signal or power conductors through the shielding boundary.
2. Prior Art
Electronic circuits operating at high frequencies or having high gain often emit or are sensitive to electromagnetic radiation. It is conventional to enclose such circuits in a conductive enclosure coupled to a circuit ground, to attenuate radiation that is emitted or received. The effectiveness of a shielding enclosure depends on its conductivity, its position relative to the shielded circuitry, the extent to which the enclosure continuously surrounds the circuits, and other factors. Improving the effectiveness of high frequency shielding thus generally requires more metal, defining a more continuous enclosure, and/or arranging the shield closer to the affected circuits. A conductive shielding enclosure may define gaps, depending on the frequencies to be shielded. However, to attenuate relatively higher frequency radiation effectively, any gaps must be correspondingly smaller.
In connection with computing and radio communications equipment, the frequencies of clock oscillators, multiplying phase locked loops and similar circuits may be quite high. The basic clock speed of a conventional portable computer, for example, typically ranges from 25 to 66 MHz, and advances are pushing the clock speeds upwardly. In addition, operations at different frequencies produce harmonics at the sum and difference of the frequencies, and in the portable computer example, harmonics up to 900 MHz or more may be present and may require shielding. This requires a very intensive shield, i.e., a nearly complete enclosure that is quite conductive and is placed close to the circuits that emit or are sensitive to the electromagnetic radiation.
The most typical shielding enclosure is a conductive box of continuous sheet metal, built of a plurality of rigid panels forming a rectilinear box around the affected circuits. The external housing of electronic equipment may include means forming a conductive box, and various subassemblies inside the box may be similarly enclosed in smaller boxes to prevent the subassemblies from inducing currents in one another. A number of problems are encountered in the manufacture and assembly of electronic equipment due to the need for such shielding, particularly in equipment based on printed circuits, in compact and/or light weight portable equipment and the like.
One problem is weight. It is not unusual for an electronic circuit to include various subassemblies that are separately shielded to prevent them from adding to the emissions of the assembly as a whole. Shielded subassemblies are likewise often provided within shielded subassemblies. For example, a portable computer may have an external housing having a sheet metal liner for shielding the housing, a switched mode power supply assembly that is separately shielded, a disc drive that has internally shielded driver and motor circuits within a sheet metal box, separate shielding associated with the keyboard or the display, and an internal shield or ground plane associated with the main printed circuit board. There is a need to optimize shielding to reduce weight. If the outer shielding could be optimized and made continuous, internal shielding could be reduced to the minimum needed to avoid cross coupling between subassemblies.
Another expense and complexity of manufacture is mounting of the shielding panels. For compactness, the sheet metal panels are carefully shaped to fit closely over the shielded components. Typically, the sheet metal panels have flanged edges with aligned holes for screws, rivets, bendable tabs and similar attachment means intended to permit assembly with other panels and with circuit card and housing structures, to form a self supporting metal box. This requires a plurality of complex shapes, fittings and assembly procedures for the shielding box and for the housing. The panels and their assembly and mounting must be handled with care to ensure a continuous close fitting enclosure that does touch or short circuit elements. Often it is necessary not only to provide a formed conductive panel, but also to attach standoffs or sheets of plastic or other electrical insulation over the conductive panel to prevent shorting. All these techniques add to the expense and weight of the unit.
It is known to attempt to alleviate shielding problems by making a plastic, normally-nonconductive housing into a conductive box by applying a conductive additive to the plastic, by applying a metallic coating layer, or by laminating the housing from alternating conductive and nonconductive sheets. These techniques are disclosed, for example, in U.S. Pat. Nos. 5,137,782--Adriaensen, et al. (embedded wires); 5,164,542--Hart (laminated wire screen); 5,170,009--Kadokura (electrodeposited coating); and 5,226,210--Koskenmaki, et al. (conductive paint coating). Such techniques are helpful as to shielding, but add to manufacturing complexity, particularly if the internal surface facing the shielded circuitry must be nonconductive to avoid shorting, and must be free of gaps, for example caused by scratching a coating.
According to an aspect of the present invention, shielding is provided in the form of a flexible nonconductive envelope with embedded conductive fibers sufficient to provide a low resistivity as needed for electromagnetic shielding, i.e., on the order of 10.sup.-1 to 10.sup.-2 .OMEGA./.quadrature., and a nonconductive surface facing toward the shielded circuit. The envelope is flexibly arranged around the shielded circuit, and can have passages for conductors and/or access, providing the advantages of rigid panel shielding without the manufacturing and assembly problems.
Flexible plastic bags with conductive coatings on an exterior side are known in connection with shipping bags intended to avoid damage to sensitive CMOS integrated circuits and the like due to electrostatic discharge. A person handling the protected circuit first touches the conductive bag, whereupon any potential difference due to static electricity is equalized, sometimes with the occurrence of a discharge or spark. Protection from electrostatic discharge is unlike high frequency shielding in that the energy of a static electricity discharge, occurring for example with manual handling, is normally very low. The discharge occurs at a point and is characterized by relatively low frequencies. The circuit is normally not operational, but is packed for shipment. The objective is simply to prevent the discharge from damaging the thin semiconductor junctions of the CMOS circuits.
Protective shipping bags for circuit cards accordingly have a very thin external metallized layer to dissipate static discharges, normally so thin that one can see through readily. The metallization is minimal because attenuation of the discharge by 20 dB is adequate for circuit protection. Electrostatic discharge packaging functions adequately for its purposes even when the packaging leaves gaps open at closures or perhaps where the metallization is scratched or worn. Normally, however, the open end of the bag extends beyond the enclosed circuit card and is folded over and taped, leaving no passages to the circuit card without touching and removing the bag.
It is known to provide electrostatic discharge protection as a permanent feature of a circuit card mounting, for example as in U.S. Pat. No. 5,005,106--Kiku, where such a structure protects the circuitry of an integrated circuit bank card or smartcard from electrostatic discharge, and resides in a plastic housing with the circuitry. This unit is self contained, and thus the electrostatic discharge material does not interfere with other aspects of circuit operation and mounting. The discharge-protective structure does not form a complete envelope around the affected circuit. Kiku uses a minimally conductive film rather than highly conductive embedded fibers as needed for shielding against electromagnetic interference. No means are provided such as an extension neck, access opening or the like for dealing with passage of conductors or access to the circuit elements.
It is also known to wrap a circuit card with a web of discharge protective shielding material as in U.S. Pat. No. 4,896,001--Pitts et al. According to Pitts, an electrostatic discharge material in the form of a web is provided on the high end of the conductivity range for electrostatic discharge protection. This effectively compromises electrostatic discharge protection to improve shielding against interference, and compromises shielding effectiveness to improve electrostatic discharge protection. Whereas the web is wrapped over itself and around the circuit card, the circuit card is not closely sealed and the result is not a complete envelope forming a full shielding barrier.
Another possibility is to cure nonconductive and conductive materials in alternating layers directly on the circuit card elements. However, this precludes any further access to the circuit card elements and tends to confine heat. High frequency devices, such as the processor of a computer, tend to dissipate substantial heat. It also is desirable to reserve access to the circuit card, for example to allow a math coprocessor circuit to be added or to allow the processor chip to be replaced, via a zero-insertion force (ZIF) mounting arrangement.
For electromagnetic shielding of circuits that are sensitive due to high gain and/or operation at high frequencies, as well as circuits that emit the offending high frequency electromagnetic radiation, a great deal more conductive material and a more complete conductive enclosure are needed for shielding against electromagnetic interference (EMI) than for protecting against electrostatic discharge (ESD). An adequate shield for EMI purposes should attenuate radiation at 50 to 60 dB over a frequency range of 30 MHz to 1.0 GHz. The shield must be substantially continuous and must enclose the shielded circuits closely and completely, i.e., forming a closed highly conductive envelope. Whereas electrostatic discharge packaging may require a surface resistivity of 10.sup.+1 to 10.sup.+12 .OMEGA./.quadrature., electromagnetic shielding has a surface resistivity of 10.sup.-1 to 10.sup.-7 .OMEGA./.quadrature..
It would be advantageous to provide a flexible enclosure that is more intensively conductive, for use as a shield in place of rigid sheet metal panels and boxes. The present invention concerns a low-resistivity flexible shielding envelope that can be mounted in a device such as a portable computer or the like, having an insulated side facing the circuit and a conductive side facing the envelope, and with access provided to the circuit and/or means available for passing signal lines or power lines through the shield barrier while maintaining a complete shield barrier.